Steering stabilizer with valved accumulator

ABSTRACT

A steering linkage assembly ( 10 ) having a resistance assembly ( 30 ) for resisting turning movements of a steerable member ( 17 ), and a trim assembly ( 28 ) for remotely adjusting the length of a linkage ( 10 ) between the steerable member ( 17 ) and a frame member ( 12 ). A trim valve ( 52, 52′, 52,″ 301 ) is operable between closed and open positions for allowing a trim piston ( 36 ) to move from one locked position (C 1 ) to another locked position (C 2 ) to change the length of the linkage ( 10 ). A liquid pressurized by a gas pressure source ( 88 ) is provided to the resistance assembly ( 30 ) and to the trim assembly ( 28 ) by a single pressure accumulator assembly ( 275 ), or by a dual accumulator assembly ( 75, 175 ) providing liquid at a trim pressure and a resistance pressure. The stroke of the trim piston ( 36 ) is sufficient for the fluid systems ( 28, 30, 75, 175, 275 ) to remain pressurized at all times.

RELATED APPLICATIONS

[0001] This is a continuation of my prior PCT International Applicationof the same title filed Apr. 15, 2002, and designating the U.S., whichis a continuation of my prior U.S. patent application Ser. No.10/105,868 filed Mar. 25, 2002, which is a continuation-in-part of myprior PCT International Application Serial No. PCT/US02/04449 filed Feb.15, 2002, and designating the U.S., which is a continuation-in-part ofmy prior U.S. patent application Ser. No. 09/699,520 filed Oct. 31,2000, the entire contents of these four applications being incorporatedherein by reference.

FIELD OF THE INVENTION

[0002] This invention relates to vehicle steering systems and moreparticularly to a device for holding the steerable wheels of a vehicle,such as a motor home, bus, truck, automobile or the like, so that acenter steering position is maintained in spite of spurious steeringinputs, such as those caused by variable crosswinds, crown curvature orslant of the highway, or other factors tending to adversely affectvehicle steering by the driver.

BACKGROUND OF THE INVENTION

[0003] The steering systems of highway vehicles and the like aredesigned primarily for driver control. In these systems, the steeringforce required on the steering wheel and the ratio between steeringwheel movement and movement of the steerable ground wheels depend uponthe characteristics of the particular vehicle and the conditions underwhich it will usually be operated. A wide variety of extraneous forcescan act on a vehicle steering system and spurious steering inputs causedby these forces must be dealt with satisfactorily in order to providestable and controllable steering of a vehicle. As vehicle speedincreases, the effects of any spurious steering inputs are magnified,making it necessary for the driver to exercise more precise and carefuldriving control.

[0004] Vehicles with steering systems having positive caster generallytrack relatively straight ahead and generally resist normal steeringinputs away from center, including those of the driver. Intentionalturning maneuvers by the driver therefore require sufficient turningforce to overcome this positive resistance to movement away from center.When the driver relaxes the turning force applied to the steering wheel,a positive caster system has a definite tendency to return to itsstraight ahead position, although it may overshoot the neutral or centerposition if the steering wheel is entirely released.

[0005] While positive caster is desirable in some respects, it is notwithout compromises over the full steering spectrum. For example, theadverse effects of strong gusty cross winds are usually more pronouncedwith large amounts of positive caster. As its name would imply, thevehicle tends to caster towards the side of the roadway to which it isbeing pushed by the wind. Thus, the adverse steering inputs caused bycrosswinds are directly related to the amount of positive caster offset,which is a classic example of having to balance a benefit with adetriment. The small amount of stability gained from castering thesteerable wheels on a non-windy day may be paid for many times over whendriving in a crosswind because of the destabilizing effect of thecrosswind caused by positive caster offset. Positive caster offset alsoallows steering inputs from rutted and other imperfect roadway surfacesto steer back against the driver and thereby cause road wander, which isa universal driving complaint, particularly by driver's of heavyvehicles such as trucks and motor homes. Similarly, a high crown at thecenter of the roadway or a slanted roadway can cause vehicles to turntoward the edge of the roadway, that is, in the downhill direction. Inaddition, generous positive caster provides significant resistance tosmall radius turns, which can make city driving quite fatiguing. Theseadverse effects are some of the negative aspects of achieving steeringstability through generous amounts of positive caster.

[0006] For the lack of a more advanced method, steerable wheel casteringhas been accepted by the industry as a low-cost method of achievingsteerable wheel returnability. Accordingly, many over-the-road vehiclesare provided with generous amounts of positive caster. Not much thoughthas been given to the self-defeating side effects of steerable wheelcastering. The failure of the industry to recognize the critical need toprovide directional stability by replacing steerable wheel casteringwith another method of achieving steerable wheel returnability may godown in history as one of the longest enduring vehicle designoversights.

[0007] Thus, a highly important consideration that has long beenoverlooked by the industry is that steerable wheel castering is directlyresponsible for road wander, crowned road steering wheel pull and crosswind steering problems. Keeping a vehicle tracking straight and undercontrol currently requires an inordinate amount of driver steeringcorrections to counteract the adverse side effects of castered wheels.The repetitive task of making thousands of precise steering correctionsmile after mile weighs heavily on a driver's physical and mentalwell-being, and may result in extreme driving fatigue. Thus, vehicledirectional stability can best be achieved by stabilizing the on-centerbehavior of the steerable wheels with a more suitable method than thetraditional steerable wheel castering used on many current productionvehicles.

[0008] Another drawback of prior art steering systems is that spuriousinputs transmitted from the roadway through the steerable wheels affectsubstantially the entire steering assembly before encountering anystabilizing resistance from the steering wheel. The transmission ofthese inputs between the steerable wheels and the steering wheel causesthe interconnecting components of the steering system to repeatedlyoscillate between states of tension and compression. Such oscillationscause wear and slack in ball joints and other connections and have longbeen considered a primary source of stress fatigue which can lead topremature failure of various steering system components. Mechanicalslack due to worn parts can also be a cause of steering systemoscillations and vehicle wandering that require constant corrections andtherefore produce driver fatigue.

[0009] The ideal driving situation is therefore one where the steeringsystem inherently causes the vehicle to travel in an unswerving straightline unless the driver intentionally turns the vehicle in anotherdirection. Thus, the ideal steering system would require relativelylittle attention from the driver as the vehicle progresses along astraight line path down the roadway. From a steering standpoint, thevehicle should not respond to anything but the driver's steeringcommands and these must be of sufficient magnitude to overcome asignificant resistance to turning away from center. In the absence of asteering input by the driver, the vehicle should literally do nothingbut progress straight ahead.

SUMMARY OF THE INVENTION

[0010] The invention provides a center stabilizer assembly for improvedon-center holding of the steerable wheels, and significantly reducesdriver fatigue because it results in a major reduction in driversteering inputs. The stabilizer assembly is easily activated by thedriver while driving the vehicle, and its activation makes driving morepleasurable and less fatiguing. The stabilizer assembly compriseslinkage means of variable length that extends between the steerablewheels and an axle or frame member such that the length of the linkagemeans defines the center position of the steering system. The linkagemeans comprises a resistance assembly that provides a resistance forcefor resisting steering forces tending to move the steerable wheels toeither side of the center position, and a trim assembly for changing thecenter position to be maintained by the resistance assembly.

[0011] The trim assembly comprises a trim piston, a trim cylinderproviding first and second trim chambers one on each side of the trimpiston, and a trim fluid system for providing a flow of fluid to andfrom each of the trim chambers. The trim fluid system includes trimconduits, a fluid accumulator and a remotely actuated trim valve forcontrolling trim fluid flow. The trim valve is operable between a closedposition for preventing the fluid flow to hold the trim piston in alocked position, and an open position for allowing the piston to move toa new locked position in the trim cylinder. Movement of the trim pistoncauses fluid flow to one of the trim chambers and fluid flow from theother of the trim chambers. This fluid flow permits the length of thelinkage assembly to change relatively freely in response to steeringforces, which in turn permits the steerable wheels to move freely to anew center position in response to an applied steering force.

[0012] The resistance assembly includes a component that moves with thesteering system in response to steering wheel movement, and resistanceto movement of this component provides a resistance force opposingmovements of the steerable wheels to either side of their centerposition. These movements include large radius turns that occur when avehicle is steered through maneuvers at highway speeds and small radiusturns that occur when a vehicle turns a corner. During large radiusturns, the resistance assembly provides a resistance force that biasesthe steerable wheels back toward their center position, and this biasserves as a return force to return the steerable wheels to their centerposition upon removal of the steering force producing the large radiusturn.

[0013] More specifically, the resistance assembly comprises a resistancepiston means, a resistance cylinder providing first and secondresistance chambers one on each side of the resistance piston means, anda resistance fluid system for providing a flow of fluid to and from eachof the resistance chambers. The resistance fluid system includesresistance conduits and a fluid accumulator. The accumulator may be thesame as or separate from the trim accumulator. In the latteralternative, a separate resistance accumulator forms a dual accumulatorassembly with the trim accumulator. The term “conduit” is used in thisspecification as a generic term to refer collectively to lines, pipes,tubes, hoses, passages, passageways, ports and/or the like for conveyingand/or communicating a fluid.

[0014] In several of the embodiments described herein, a separateresistance accumulator is arranged concentrically around the trimaccumulator and both are housed in the same casing. The concentricresistance accumulator may be operated at a higher pressure than theinner trim accumulator, and the latter is preferably pressurized byproviding a fluid flow path from the former to the latter through apressure regulator. In these embodiments, the remotely actuated trimvalve of the trim assembly is located either internally within anintermediate head between the trim assembly and the resistance assembly,or alternatively within the separate trim accumulator. In a furtherembodiment, the trim and resistance assemblies use a single jointaccumulator, and the remotely actuated trim valve is located in thisjoint accumulator.

[0015] During small radius turns, the resistance assembly may berendered ineffective by the trim assembly to permit easy away fromcenter movements during such turns. This is accomplished by causing thetrim valve to open, which in turn allows movement of the trim pistonaway from its locked position in response to the steering force appliedto produce the small radius turn. For this purpose, the control systemincludes a remote switch for turning a solenoid actuator of the trimoperating system on and off remotely from the driver's station of thevehicle. When the remote switch is in its ON position to activate thesolenoid actuator, the trim valve is opened so that no resistance forceis applied to the steering system by the linkage assembly because thetrim piston is not “locked” and is therefore free to move away from itspreviously locked position in the trim cylinder. Operation of the sameremote switch to its ON position during normal straight ahead travel ofthe vehicle also allows the centered position being maintained by theinvention to be changed, i.e., “trimmed”, during vehicle operation.

[0016] Internal passages and/or orifices in an intermediate head1between the trim cylinder and the resistance cylinder may be sized toprovide a dampening action sufficient to prevent overly rapid movementsof the trim piston away from its previously locked position, such asmight otherwise occur during the blowout of a tire on a steerable wheelwhile the trim valve assembly is open. Similar internal passages and/ororifices in an intermediate head also may be sized to provide adampening action sufficient to prevent overly rapid movements of theresistance piston away from its centering position. The invention maythus provide a relatively high degree of protection against a loss ofvehicle steering control due to tire blowouts or other accidentalimpacts to a steerable wheel.

[0017] The stabilizer includes means for remotely and selectivelyvarying both the amount of resistance to movement away from center andthe preselected position of the steerable member relative to the vehicleframe. Both of these remote adjustments can be made by the driver whilethe vehicle is in operation. A “manual” control system may be employedthat uses a manually operated remote switch for actuating the trimsolenoid and a manually adjustable pressure regulator for varying boththe trim system and resistance system pressures. For this alternative,the manual solenoid switch, an air pressure gauge and a manual regulatordial are preferably located at the driver's station of the vehicle.

[0018] The level of resistance to movement away from center may beremotely adjusted either by such a manual control system operable by thedriver or by a microprocessor control system responsive to the speed ofthe vehicle. Thus, the turning resistance of the present invention isreadily adjustable to provide a low level at low speeds and a high levelat speeds of about 35 mph or greater. In this regard, the steeringstabilizer of the present invention is much less complex than prior artarrangements, such as those which combine high positive caster near thecenter position and complex power steering systems for varying the levelof power assist from a low assist level for large radius turns to a highassist level for small radius turns.

[0019] The centering return force provided by positive wheel casterfollows a force curve that may provide relatively little, if any,turning resistance in the straight ahead position or for large radiusturns immediately adjacent to the straight ahead position. Theresistance pistons may be sized to provide a resistance force whichblends with any return force provided by the normal geometry of thefront end of a motor vehicle. The invention can increase substantiallythe turning resistance available at and immediately adjacent to eitherside of the straight ahead position of the steerable wheels. The turningresistance provided by the invention at or near the centered wheelposition should be sufficiently large to resist spurious steering inputsgenerated either by the driver or by an overactive power steeringsystem.

[0020] In a preferred embodiment, the piston sizes and other stabilizerparameters are chosen so that a total break away steering force of atleast 50 pounds, preferably at least 100 pounds, and more preferably atleast 200 pounds for automobiles or light trucks and 300 pounds forheavier vehicles such as motor homes and 18 wheel trucks, must beapplied to the tie rod in order to initiate break away turning movementof the steerable wheels at vehicle speeds above about 35 miles per hour.For city driving at vehicle speeds of about 35 miles per hour or less,the break away force required may be lowered to about 100 pounds, morepreferably below about 50 pounds, at the tie rod.

[0021] At greater turning angles (small radius turns), the resistanceforce provided by the invention may be nullified as positive casterreturn force increases. Thus, where the steerable wheels are providedwith a generous amount of positive caster as the turning angleincreases, which is often the case with highway motor vehicles, it maybe desirable to activate the solenoid actuator of the trim valve andthereby release the trim piston so that no further turning resistance isprovided by the resistance piston, at least over a predetermined rangeof turning angles such as, for example, greater than 3 degrees to eitherside of center. However, in some applications where there is little orno positive caster, the resistance force need not be nullified, butinstead the resistance piston and cylinder may be sized to provide aresistance force effective over the entire range of turning angles,which for highway vehicles is usually limited to 45 degrees on eitherside of the straight ahead position (the “O” position). Thus, dependingon the amount of positive caster, the resistance force may be desirableover a range of 0-45 degrees or 0-10 degrees or 0-5 degrees or 0-3degrees on either side of center, these being only a few examples.

[0022] The stabilizer is preferably connected between the steeringsystem and the front axle or a nearby frame member of the vehicle in aposition that allows the steerable member(s) to move through its fullrange of steering movements while providing sufficient leverage for theapparatus to resist movement of the steerable member away from thecenter position producing straight ahead travel of the vehicle. Thesteering system connection may be made to any steering system componentproviding appropriate range and leverage, such as a tie rod which joinsthe two front steerable wheels of a highway vehicle, or the pitman armconnected to the reduction gear. The frame connection may be made to anycomponent serving as a fixed mounting relative to the steering system.

[0023] The invention may be used with steering systems having areduction gear between the steering wheel and the steerable wheels. Inthis application, the stabilizer is preferably connected to the steeringsystem at a location between the steerable wheels and the reduction gearso as to be unaffected by any slack in the reduction gear or incomponents and connections between the reduction gear and the steeringwheel. It is therefore on the slow side of the reduction gear ratio. Theinvention thus provides a zero backlash center stabilizer assembly.

[0024] The level of steering force required to initiate or breakawayinto a steering movement away from center is sometimes referred to inthis specification as the “break away resistance”. Different levels ofbreak away resistance and of resistance force may be appropriate tocompensate for changes in the forces acting upon the vehicle. Thus, theresistance force provided by the invention may be increased or decreasedto provide a level of force sufficient to overcome any spurious steeringinputs and to suit driver road feel, particularly a feel of the steeringwheel that lets the driver know when the steered wheels are beginning tomove away from center and are closely approaching return to center. Inother words, the invention provides a distinctive feel when approachingor leaving the center position. Thus, the sense of touch is added to thevisual sense to aid control of the vehicle and reduce driver fatigue.

[0025] In the absence of the invention, spurious inputs to and/ormechanical slack in the steering assembly require almost constantmanipulation of the steering wheel by the driver and make it almostimpossible for the driver to hold the vehicle on a true straight aheadcourse. Use of the invention therefore permits a substantial reductionor elimination of the caster angle of vehicles with positive caster,thereby significantly reducing the crosswind effect and providing thedriver with a positive touch control not heretofore attainable withpositive caster. Positive stability is thereby achieved for previouslyunstable steering systems. In addition, less manipulation of thesteering system provides a substantial reduction in tire wear,particularly for large vehicles. It is estimated that use of theinvention on large trucks in the United States alone may reduce tirereplacement costs by as much as 3 billion dollars over a five yearperiod.

[0026] Although the present invention is particularly useful as a centerstabilizer assembly for motor vehicles, it can be employed to hold thecenter position of any steerable member moveable to either side of apreselected position. For example, the stabilizer can keep an outboardmotor centered so that a boat follows a straight course over the waterin the presence of spurious steering forces produced by wind and waveaction. The stabilizer can also be used to keep centered such steerablemembers as the rudders of ships or airplanes and the tongues of tandemtrailers or railway cars. The stabilizer is useable with both power andnon-powered steering systems, with the level of resistance forcesprovided usually being less for vehicles without power steering.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The invention, both as to its structure and operation, may befurther understood by reference to the detailed description below takenin conjunction with the accompanying drawings in which:

[0028]FIG. 1 is a plan view illustrating installation of the inventionbetween the frame and steering system of a motor vehicle;

[0029]FIG. 2 is a schematic diagram of the fluid and electrical systemsand of the major components of the invention, and includes sectionalviews showing structural details of the steering trim assembly, theturning resistance assembly and the valved accumulator assembly;

[0030]FIG. 3 is top partial sectional view of the valved accumulatorassembly of the invention;

[0031]FIG. 4 is an enlarged transverse sectional view of the trim valvesection of the trim cylinder assembly taken along line 4-4 of FIG. 2;

[0032]FIG. 5 is a fragmentary sectional view of the trim valve sectiontaken along line 5-5 of FIG. 4;

[0033]FIG. 6 is a fragmentary sectional view of the trim valve sectionsimilar to FIG. 5, but with the valve in its open position

[0034]FIG. 7 a fragmentary sectional view of the solenoid operated airvalve 109 in FIG. 2;

[0035]FIG. 8 is a schematic diagram of the fluid and electrical systemsand of the major components of a modification of the invention, andincludes sectional views showing structural details of the steering trimassembly, the turning resistance assembly and the valved accumulatorassembly;

[0036]FIG. 9 is a schematic diagram of the fluid and electrical systemsand of the major components of another modification of the invention,and includes sectional views showing structural details of the steeringtrim assembly, the turning resistance assembly and the valvedaccumulator assembly;

[0037]FIG. 10 is top sectional view of the valved accumulator assemblytaken along line 10-10 of FIG. 9;

[0038]FIG. 11 is an enlarged sectional view of the valved accumulatorassembly taken along line 11-11 of FIG. 10;

[0039]FIG. 12 is a schematic diagram of the fluid and electrical systemsand of the major components of a further modification of the invention,and includes sectional views showing structural details of the steeringtrim assembly, the turning resistance assembly and the valvedaccumulator assembly;

[0040]FIG. 13 is top sectional view of the valved accumulator assemblytaken along line 13-13 of FIG. 12; and,

[0041]FIG. 14 is an enlarged sectional view of the valved accumulatorassembly taken along line 14-14 of FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION

[0042] The steering stabilizer of the present invention comprises alinkage assembly, generally designated 10, which may be connectedbetween a front axle or frame member 12 and the tie rod 14 of aconventional motor vehicle as shown in FIG. 1 of the drawings. Steeringinputs by the driver are transmitted to the tie rod 14 and the steerablewheels 17,17 by the pitman arm (not shown) of the vehicle's steeringgear. The outer end of a resistance rod 16 of linkage assembly 10 isconnected to the tie rod 14 by means of a ball joint 18 connectedbetween the outer rod end and tie rod 14 by a bracket 20.

[0043] The outer end of a trim rod 22 at the opposite end of linkageassembly 10 is connected to the axle 12 by means of a ball joint 26connected between the outer rod end and axle 12 by a bracket 24. Theball joints 18 and 26 permit pivotal movement in the horizontal planeand to a limited extent in the vertical plane, and are conventionaljoints wherein an enlarged spherical ball is mounted by a stub on thecorresponding bracket and is held for pivotable movement within asurrounding journal structure carried by the corresponding rod end.

[0044] The components of the stabilizer system and the way in which theyhold center during vehicular travel, and return a vehicle steeringsystem to center after turning of the vehicle, will now be described. Itis to be understood that the components described are connected togetherby appropriately sized fluid conduits and electrical wires and thatthese conduits and wires are represented by the lines interconnectingthe components as shown in the drawings.

[0045] Referring now to FIGS. 1 and 2, the linkage 10 includes a trimassembly 28 and a resistance assembly 30. Trim assembly 28 comprises atrim cylinder housing 32 enclosing an interior cylinder 33, which isdivided into two chambers 34 and 35 by a trim piston 36 secured to theinner end of trim rod 22. The outer ends of housing 32 and cylinder 33are closed by a head 40 having a journaled and sealed aperture 41 forsliding passage of trim rod 22. A set of multiple seals 42 and the head40 are secured in place by being threaded into or crimped onto thecylinder housing 32. The end portion of trim rod 22 projecting beyondcylinder housing 32 may be surrounded by a dirt and grease barrier inthe form of a flexible boot, such as boot 48 in FIG. 8.

[0046] The inner end of cylinder 33 is closed by an intermediate head50, in which may be included an air operated trim valve assembly,generally designated 52, which is described below. The side ofintermediate head 50 opposite to trim cylinder 33 closes a resistancecylinder housing 53. The end of cylinder housing 53 opposite to head 50is closed by an end head 54 to provide a pair of resistance chambers 55and 56 on opposite sides of a resistance piston means, generallydesignated 57.

[0047] An accumulator assembly, generally designated 75, includes aninner low pressure section 76 defined by an inner cylindrical wall 77and an outer annular high pressure section 78 defined by an outercylindrical wall 79, the walls 77 and 79 being secured between a cover80 and a base 81. The inner section is filled with a liquid through afill tube 82 to create a low pressure gas chamber 83 and a trim liquidreservoir 84, and the outer section is filled with a liquid through afill tube 85 to create an annular high pressure gas chamber 86 and anannular resistance liquid reservoir 87. The chamber 86 is connected to ahigh pressure gas supply 88 via a gas line 89 and a port 90 in cover 80.An alternative to gas line 89 is the gas line 167 and its associatedvariable pressure system as shown in FIG. 8 and described in detailbelow. The low pressure chamber 83 is connected to the high pressurechamber 86 via a cover passage 91 and a pressure regulator 93 located inthe passage 91 for reducing the gas pressure from that desired inchamber 86 to that desired in chamber 83.

[0048] The high pressure chamber 86 is connected via a base passage 96,a base port 97, a line 98 and a head port 202 to a passage 99 (FIG. 5)in intermediate head 50, and head passage 99 in turn is connected toresistance chambers 55 and 56 via a port 156 leading to chamber 56 andan arcuate passage 153 and a port 154 leading to chamber 55. The lowpressure chamber 83 is connected via a base passage 101, a base port102, a line 103 and a head port 208 to a passage 104 in intermediatehead 50, and head passage 104 in turn is connected to a valve chamber106 on one side of a valve member 112. Valve chamber 106 in turn isconnected to trim chamber 34 via a port in the form of a valve seat 100,a head passage 107 and an annular passage 69 that is defined by thehousing 32 and the trim cylinder 33 and leads to a chamber port 70. Thevalve chamber 106 is also connected to trim chamber 35 via a port in theform of a valve seat 105, a head passage 110 and a chamber port 73.

[0049] Referring now to FIG. 6, valve member 112 has an integral pistonportion 114, and chamber 106, member 112 and piston 114 are part of thetrim valve assembly 52. Although in this embodiment piston 114 is formedintegrally with valve member 112, the piston may be a separate elementconnected to the valve member by a shaft, rod or the like. Operation ofthe piston 114 and thereby valve member 112 is controlled by a solenoidactuated air valve 109 that is in a high pressure air line 108 connectedto a pressure chamber 119. The valve seats 100 and 105 are opened andclosed by the valve member 112 that is actuated to its closed positionby high pressure air fed to the pressure chamber 119 through the line108, a head port 252, a head passage 111 and a passage 113 in a valvechamber cap 131 during activation of a solenoid 60 of air valve 109 inresponse to the ON position of a remote electrical switch 62 of anelectrical control system. Valve 109 is normally held open by itssolenoid and is closed only momentarily by deactivating its solenoid tobriefly open the main valve seats 100 and 105 for making a trimmingadjustment as described below.

[0050] Although switch 62 may be hard wired to the solenoid, it ispreferable a remote switch that may be mounted on the steering wheel 38of a motor vehicle for generating a radio signal to activate thesolenoid 60 via a radio signal receiver 117. The ON position of switch62 activates a signal generating unit 118 that in turn activates thesignal receiver 117 causing it to supply an electrical current from apower source 63 to the valve solenoid 60. With this option, the switch65 of a relay 64 does not close until the receiver section 117 receivesan ON signal from the remote unit 118. On the other hand, switch 65opens in the absence of a signal from unit 118 when switch 62 is in itsOFF position for trimming the steering system. The signal generator andreceiver are of conventional design and allow the driver of the vehicleto conveniently change (trim) the centered position of the steeringsystem as desired. The solenoid operated valve 109 is secured to theaccumulator cover 80 by an adapter 58 leading to a cover passage 121connected to a cover passage 115 connecting the high pressure air sourceto the accumulator chamber 86. As may be seen in FIG. 7, internal to thesolenoid is a double ended plunger 59 having at its upper end a valveelement 125 that, when switch 62 is ON to activate the solenoid, ismoved downward by the solenoid away from a valve seat 120 to open valve109. In other words, when the solenoid 60 of air valve 109 is actuatedby an electric current supplied thereto by the radio receiver 117 inresponse to the electrical switch 62 being in its ON position, theplunger 59 compresses spring 61 and causes valve element 125 to openvalve seat 120. A differential between high and low pressure air willthen act on the trim valve piston 114 and force the trim valve member112 to close the trim valve seats 100 and 105.

[0051] In the absence of electrical power to the solenoid 60, i.e., whenthe switch 62 is in its OFF position, the solenoid plunger 59 is pushedupward by the compressed return spring 61 so that valve element 125closes the valve seat 120 and thereby the flow path for pressurized airto reach the valve piston 114. When valve seat 120 is closed, thechamber 119 adjacent the piston 114 is vented to the atmosphere via anadapter and solenoid passage 122 and a vent passage 123 through solenoid60. The venting of chamber 119 allows the differential between trimsystem pressure and ambient, which may be aided by a compression spring94, to open the valve seats 100 and 105 by pushing away therefrom thevalve member 112. The trim valve assembly 52 is thereby actuated to itsopen position. When valve seat 120 is reopened to actuate the valveassembly 52 to its closed position, the vent passage 123 is closed by avalve element 126 at the lower end of solenoid plunger 59.

[0052] To insure that there is no fluid communication from the highpressure section to the low pressure section and vice versa, a belloframdiaphragm 129 extends completely over and seals the high pressure sideor actuating surface of the valve piston 114 (FIGS. 5 and 6). Thediaphragm 129 has a rib 130 around its outer edge and is held in placeby clamping this rib in an annular groove 127 between the intermediatebead 50 and the chamber cap 131 providing the closure for the pressurechamber 119. The diaphragm also includes an annular loop segment 132that permits reciprocation of the valve piston while maintaining thedesired seal between the valve piston 114 and the air passage 113 in cap131, passage 113 communicating with head passage 111 as shown in FIG. 6.An annular U-cup seal 133 around the valve piston 114 insures that thereis no fluid communication from the valve chamber 106 to the piston sideof the diaphragm 129 when the pressure chamber 119 is opened to the ventpassage 123. An air breather vent 19 insures that the space between theseal 133 and the diaphragm 129 remains open to ambient pressure.Appropriate O-ring seals, as represented by small black or cross-hatchedcircles in the drawings, are provided between the various components ofthe invention across which there may be a pressure differential.

[0053] When trim valve assembly 52 is in its open position with solenoid60 deactivated (valve seat 120 closed and valves seats 100 and 105open), a fluid, preferably a liquid, is supplied from the trim reservoir84 to the trim chambers 34 and 35. Therefore piston 36 is relativelyfree to move back and forth in trim chambers 34 and 35 away from itslast locked position Cl, in which piston 36 was “locked” when trim valveassembly 52 was last closed (valve seat 120 open and valve seats 100 and105 closed). The availability of an accumulator reservoir, such as trimreservoir 84, is required even though the fluid may flow back and forthbetween trim chambers 34 and 35 via the interconnecting passages whentrim valve assembly 52 is open. This is because of the volume of chamber34 taken up by the presence of trim rod 22, which makes the volumechange in chamber 34 caused by the movement of piston 36 different fromthe volume change in chamber 35 caused by such movement.

[0054] Also shown in FIGS. 1 and 2 is the resistance assembly 30 havinga cylinder means containing a piston means 57 comprising a dual pistonarrangement. Although a single piston arrangement is possible, thiswould require a more complicated valving arrangement. The resistancecylinder housing 53 encloses two separate interior cylinders 150 and151, the adjacent ends of which are connected together by a collar 152.The cylinders 150 and 151 contain resistance pistons 142 and 144,respectively, and these components together define respective centeringor resistance chambers 55 and 56. An enlarged rod head 140 is keyed andfastened to the inner end of a resistance rod 16. Head 140 is positionedbetween opposing faces of pistons 142 and 144 and serves as the actuatorfor these pistons.

[0055] A breather port 147 is provided in collar 152 and vents toambient to allow air to enter and leave the space S between pistons 142and 144 as they reciprocate in cylinders 150 and 151. Alternatively, thespace S may vent to ambient via a breather line or to a vent reservoir(not shown). The end portion of resistance rod 16 projecting beyondcylinder end head 54 is connected to the mounting bracket 20 by theconventional ball joint 18, and also may be surrounded by a dirt andgrease barrier in the form of a flexible bellows similar to bellows 48shown for trim rod 22 in FIG. 8.

[0056] The end of cylinder 150 opposite to collar 152 is closed by ahead 54 having a journaled and sealed aperture 49 for sliding passage ofresistance rod 16. A set of multiple seals 51 and a journal 146 aresecured in place by a snap ring 48, and the end head 54 is threaded intoor crimped onto the cylinder housing 53. Piston 142 has a sealedaperture 69 for sliding passage of rod 16 during its movement of piston144. A piston rod seal 71 and a journal 70 are secured in piston 142 bya snap ring 71. The end of cylinder 151 opposite to collar 152 is closedby the adjacent side of the intermediate head 50, and the two cylinders150 and 151 are secured to the collar 152 and the opposite respectiveclosures by crimping at positions beyond seal rings 44, 45, 46 and 47.

[0057] Pistons 142 and 144 are shown in FIG. 2 in their rest positionscorresponding to a centered steering system. These rest positions areshown offset toward the intermediate head 50 (to the left in FIGS. 2 and9) in order to equalize the volumetric capacity of chambers 55 and 56since chamber 55 also contains a portion of rod 16. Both pistons arearranged for compressive movement toward the opposite ends of theirrespective chambers, piston 142 traveling in chamber 55 and piston 144traveling in chamber 56 within cylinders 150 and 151, respectively. Acompressive movement of piston 142 to the right is shown in FIG. 8 byway of example.

[0058] The rest or retracted position of each piston is defined by theinternal annular collar 152 which serves as a piston stop and preferablyhas an axial width substantially equal (preferably within one-tenthousandth of an inch) to the axial thickness of rod head 140. A stopwidth greater than the head thickness is undesirable because gapsbetween opposing surfaces would allow unbiased movement (slack) betweenrod 16 and cylinders 150 and 151. A stop width less than the headthickness is also undesirable because this would let fluid flow back andforth between chambers 55 and 56 through the connecting arcuate passage153 so that the pistons 142 and 144 would move in tandem together(drift) until one of them bottoms out against the collar 152, therebeing no centering bias applied to the rod head during such joint pistonmovement.

[0059] Therefore, the high pressure section 78 of accumulator assembly75 constantly biases the resistance pistons 142 and 144 intosubstantially simultaneous engagement with both the stop collar 152 andthe piston rod head 140 at all times while the resistance assembly ispressurized and in its centering position, such that there is nosignificant slack or drift at any time during its operation. Near theend of each centering chamber opposite to the retracted piston positionis a port for communicating fluid pressure to the chamber, port 154serving chamber 55 and port 156 serving chamber 56. Ports 154 and 156are connected together by the arcuate passage 153, which includes acollar passage 148, so that both ports communicate with chamber 56,which in turn is connected via head passage 99 and conduit 98 toaccumulator high pressure chamber 86.

[0060] The vehicle steering system is properly centered when pistons 142and 144 abut collar 152. In order to move or break away from collar 152,these pistons must overcome the resistance provided by accumulatorpressure acting through the conduit 98, which may contain an in-linefilter (not shown). A preferred feature of the fluid resistance assembly30 is the difference in diameters between piston 142 and its cylinder150 on the one hand and piston 144 and its cylinder 151 on the otherhand, the diameter of piston 142 and cylinder 150 being larger by anamount sufficient to produce equal resistance forces on pistons 142 and144 in spite of the area of piston 142 lost because the resistance rod16 passes through aperture 69 in piston 142. Thus, to provide equalworking areas, the cross-sectional area of cylinder 150 may be greaterthan the cross-sectional area of cylinder 151 by an amount sufficientfor the annular surface area of piston 142 to be equal to the disksurface area of piston 144, the difference in the cylindercross-sectional areas being equal to the amount of piston area lost byreason of rod aperture 69 in piston 142. However, for reasons ofsimplicity and economy of manufacture, the diameters of the resistancepistons and cylinders may be equal, as illustrated in FIG. 12, where thedifference in resistance forces on pistons 142 and 144 does notsignificantly affect performance or the feel of the steering wheel whenturning away from center.

[0061] Although a gas such as air could be used for the resistance fluidin chambers 55 and 56, a liquid resistance fluid is preferred because itis substantially incompressible as compared to a gaseous fluid, andtherefore provides the capability of a viscous dampening action as theliquid flows through a restricted passage, such as that provided by thepassage 99 in head 50, which may be sized to function as an orifice.Another important feature of the invention is that viscous dampeningaction may be provided by including orifices in or adjacent to the trimvalve seats 100 and 105 so that this action will be provided by movementof the trim piston in the trim chambers 34 and 35 when the trim valve 52is in its open position because solenoid 60 is deactivated, such as formaking small radius turning maneuvers.

[0062] In this regard, the length of trim cylinder 33 may be such thatthe full stroke of trim piston 36 is available for most or all smallradius turns. The smallest radius turn is defined as turning thesteerable wheels from “lock to lock”, which is usually equivalent to aturning angle of about 45 degrees to either side of center, i.e.,movement of the wheels through about 90 degrees in going from a fullright turn to a full left turn or vice versa. For many practicableapplications, the stroke of the trim piston preferably provides turningangles of at least about 15 degrees, more preferable at least about 25degrees, and most preferable at least about 35 degrees.

[0063] Another way to select the stroke of the trim piston is as apercentage of the maximum turning angle of the steerable wheels, whichis predetermined by the steering system of each vehicle and which insome cases may be as great as 52 degrees. For the present invention whenthe trim valve is open, the available trim piston stroke is sufficientto allow without resistance a turning angle in the range of about 30% toabout 100%, preferably at least about 40%, more preferably at leastabout 50% and most preferably at least about 80%, of the predeterminedmaximum. These trim piston strokes are much greater than those of priorart systems, which are substantially less than 20% of the predeterminedmaximum turning angle.

[0064] Because of the stroke available from the trim piston when thetrim valve is open, there is no need for the gas pressure to be releasedto ambient atmosphere to easily execute small radius turns, i.e., largeturning angles, without any turning resistance from the invention. Inprior art systems utilizing a trim piston with a relatively shortstroke, the accumulator pressure was dumped to ambient when the systemwas cut off to execute small radius turns. This involved a significantdelay in returning the system to full operation because of the timerequired to repressurize the accumulator. With the present invention,the system is always “ON” and fully pressurized during small radiusturns with the trim valve open so that full turning resistance isavailable instantly upon closure of the trim valve to lock the trimpiston in place.

[0065] Gas pressure in chamber 86 acts against the liquid surface tostore fluid energy received from the hydraulic side of the system, andto maintain fluid pressure on the liquid side of the respective pistons142 and 144. Thus, the gas trapped in high pressure gas chamber 86provides a spring-like return force and this chamber may be sized suchthat the return force does not vary significantly with compressivemovement of pistons 142 and 144. The resistance reservoir 87 ofaccumulator assembly 75 should be large enough to receive the entirevolume of liquid from either chamber 55 or 56 without unduly compressingthe gas in chamber 86. However, a pressure relief valve (not shown) maybe provided in communication with the high pressure gas chamber of theaccumulator to insure an upper limit to the resistance and return forcesthat may be generated by compressive contact of the piston rod head withone or the other of the resistance pistons. The associated fill tube,which is normally closed by a cap (not shown), allows liquid reservoir87 to be filled with hydraulic fluid up to the desired level asdetermined by the height of the fill tube.

[0066] Accumulator assembly 75 allows hydraulic pressure in theresistance or centering chambers 55 and 56 to be precisely set at aselected value within a relatively wide range. Both gas chambers 83 and86 are pressurizable to a selected pressure determined by the amount ofgas pressure available from the gas source 88, and by the setting of thepressure regulator 93. The gas source 88 may be an air compressor systemfor air brakes or just an air tank with a pressure gauge and a Schraedervalve or some other type of tire valve (not shown). A Schraeder valve orthe like acts like a check valve to introduce and retain the desiredamount of air in a closed space and may also be actuated to release airin the same manner as when it is used as a vehicle tire valve.

[0067] A gas pressure control may be provided and this may simplycomprise a needle valve (not shown) in line 89 in combination with adownstream pressure gauge to indicate accumulator pressure in chamber86. Alternatively, a pressure regulator may be used for maintaining amanually or automatically selected gas pressure in accumulator assembly75, such as described below in connection with FIG. 8. By varying thegas pressure in gas chamber 86 through adjusting the pressure, the breakaway resistance and the centering return force produced by the pistons142 and 144 can be increased or decreased as desired. The range ofpressures available in chamber 86 should be selected so that the breakaway resistance can be maintained at a relatively high level forcentering the steerable wheels when the vehicle is travelling at highwayspeeds.

[0068] For lighter vehicles, such as automobiles and pickup trucks, theaccumulator pressure and other stabilizer parameters may be chosen sothat a linear break away steering force of at least about 30 pounds,preferably at least about 50 pounds, more preferably at least about 100pounds, and most preferably at least about 150 pounds, must be appliedto the tie rod by the pitman arm in order to initiate a break awayturning movement of the steerable wheels. For heavier vehicles, such aseighteen wheel trucks and motor homes, these parameters may be chosen torequire a linear break away steering force of at least about 200 pounds,preferably at least about 300 pounds, and more preferably at least about300 pounds to 400 pounds.

[0069] To achieve such break away forces at the tie rod for large radiusturns at highway speeds, the gas pressure in chamber 86 is preferably inthe range of about 100 to about 300 psig, and more preferably about 150to about 200 psig. The turning forces applied to the vehicle steeringsystem are thus opposed by equal turning resistances provided by thestabilizer, and these resistance forces should be maintained for atleast small turning angles away from center, preferably 0°-5°, morepreferably 0°-3°, and most preferably within one degree on either sideof center. Small turning angles correspond to large radius turningmaneuvers.

[0070] After linear movement of the rod head 40 is initiated uponbreakaway, the steering force required to sustain movement is a functionof the pressure in the accumulator, as well as of other centeringphenomena acting on the steering system, such as positive wheel caster.Also after breakaway, accumulator pressure acting on the off-centerpiston may provide a return force that is effective over the entirerange of turning angles, which for highway vehicles is usually limitedto about 45° on either side of the center wheel position (the 0°position).

[0071] The operation of the trim assembly 28 and the resistance assembly30 will now be described in more detail. When the steerable wheel ormember of a vehicle is turned to either side of its center position,either by a steering force transmitted from the steering wheel 38 orother steerable member or by spurious steering forces transmitted to thesteerable member by a roadway or the like, this steering movement isresisted by the resistance assembly 30 of the linkage assembly 10. Untilthe steering force exceeds the compression preload provided by the airpressure in high pressure chamber 86, there will be no movement of thesteerable member.

[0072] After the compression preload is exceeded, turning movements arepermitted within the range provided by the stroke of piston 142 inchamber 55 and the stroke of piston 144 in chamber 56 by application ofsufficient steering force such as, for example, up to a steering forceof about 350 lbs. as applied to the tie rod 14. This maximum steeringforce is opposed by an equal amount of resistance force provided by thegas pressure in chamber 86, which biases the ball joint 18 and theentire steering system back toward their center positions. Small turningmovements requiring a relatively high steering force are sufficient formaneuvering a motor vehicle at highway speeds, and the resistance forcebias readily returns the steering system to center after such maneuvers,which also may be referred to as large radius turns.

[0073] Referring now to FIG. 8, there is shown a modification of theinvention wherein the air actuated trim valve 52 has been replaced by asolenoid actuated trim valve 52′ having a valve member 43 that is pushedinto its closed position by an actuating member 59′ during activation ofa solenoid 60′ and is returned to its open position in a chamber 217 bya compression spring 61′ upon deactivation of the solenoid. Since thecomponents of the accumulator, trim and resistance assemblies areessentially the same as for the embodiment of FIG. 2, except for amodified intermediate head 50′, a slightly modified cover 80′ ofaccumulator 75′ and a modified trim valve 52′, the same numericaldesignations have been used for the essentially same components.

[0074] In this modified embodiment, the resistance chamber 56 isconnected to the high pressure reservoir 87 via the line 103 and apassage 195 containing an orifice 67, which provides the dampeningfunction for blowout protection as previously described. The controlassembly of this embodiment includes a control panel 189 located in ornear the driver's station of a vehicle and having an ON/OFF switch 155provided with electrical power through a fuse panel 156. Panel 189 alsoincludes a high pressure air gauge 176, a trim button 188 formomentarily interrupting electrical power to solenoid 60′, and apressure regulator 169 with a manual adjustment knob 173.

[0075] The trimming function of this embodiment operates as follows. Theinterrupting of electrical power to the solenoid 60′ in response tomomentarily opening switch 188 or cutting off switch 155 on the controlpanel 189 causes retraction of the valve member 43 into the chamber 217,thereby placing trim valve 52′ in its open position. To insure freereciprocating movement of valve member 43, chamber 217 is vented througha valve member passage 218. When trim valve 52′ is in its open position,fluid is free to flow back and forth through the internal passages inthe modified intermediate head 50′ as previously described for theintermediate head 50 of FIG. 2.

[0076] For example, with trim valve 52′ open (member 43 pushed upward byspring 61′), trim piston 36 is free to move toward the right in FIG. 8,thereby causing fluid to flow from trim chamber 35 into trim chamber 34through chamber port 73, passages 72 and 71, valve seat ports 39 and 37,passages 68 and 69, and chamber port 70; and also into low pressurereservoir 84 through chamber port 73, passages 72 and 71, valve seatport 39, passages 66 and 74, fitting 143, and line 98. On the otherhand, movement of piston 36 to the left in FIG. 8 causes fluid to flowfrom trim chamber 34 into trim chamber 35 via chamber port 70, passages69 and 68, valve seat ports 37 and 39, passages 71 and 72, and chamberport 73; and also into low pressure reservoir 84 via chamber port 70,passages 69 and 68, valve seat port 37, passages 66 and 74, fitting 143,and line 98. However, in this instance, there may be relatively littleor no flow into reservoir 84 because the volume of chamber 35 willincrease more rapidly than the volume of chamber 34 will decrease, dueto the presence of trim rod 16 in chamber 34.

[0077] Accumulator assemblies 75, 75′, 175 and 275 allow hydraulicpressure in the centering chambers 55 and 56 to be precisely varied overa relatively wide range because this hydraulic pressure depends directlyon the level of pressurization of the respective gas chambers 86, 186and 286. For brevity, only the variable pressure control system for theembodiment of FIG. 8 will be described here, with the understanding thatthis control system is equally applicable to the embodiments of FIGS. 2,9 and 12.

[0078] The gas pressure control may comprise a manual throttle valve(not shown) between conduits 170 and 167, in combination with thepressure gauge 176 to indicate the air pressure in chamber 86.Alternately, the pressure regulator 169 may be used for maintaining amanually selected system pressure. The control knob 173 is provided topermit varying the pressure settings of the regulator by hand. Byvarying the gas pressure in gas chamber 86 through adjusting pressureregulator 169, the break away resistance and the centering return forceproduced by the compensator of the invention can be increased ordecreased as desired. The range of pressures available should beselected so that break away resistance can be varied from relatively lowat low speeds to relatively high at high speeds.

[0079] As an alternative to manual adjustment, the output pressure ofregulator 169 may be adjusted by a reversible electric motor (not shown)controlled by an on-board computer 160, which comprises a microprocessor161, an encoder 162 and a decoder 163. Encoder 162 converts to digitalsignals an analog signal 164 input from a pressure sensor 165 in the gassupply conduit 167, an analog signal 168 input from a vehicle speedsensor 171, and an analog signal 172 input from a position sensor (notshown) within regulator 169. Decoder 163 converts digital controlsignals generated by microprocessor 161 to an analog signal 174 forcontrolling the reversible electric motor which adjusts the outputpressure provided by regulator 169. The gas pressure in high pressuregas chamber 86 and the resulting resistance and centering forces arethereby made automatically responsive to the speed of the vehicle toprovide “speed sensitive centering” of the vehicle's steering system. Itmay be desirable that the resistance to turning movements away from thecenter position be increased automatically as the speed of the vehicleincreases because the effects of small off-center movements in responseto spurious steering inputs increase dramatically with vehicle speed.

[0080] Although the piston 36 is relatively free to move back and forthin the chambers 34 and 35 when trim valve assembly 52′ is open, it ispreferable that the internal passages in intermediate head 50′ be sizedto provide a dampening action sufficient to prevent overly rapidmovements of the steerable member away from its previously lockedposition, such as might otherwise occur during the blowout of a tire ona steerable wheel while the trim valve assembly 52′ is open. Theinvention may thus provide a relatively high degree of protectionagainst a loss of vehicle steering control due to tire blowouts or otheraccidental impacts to a steerable wheel. As a safety feature and toprovide such dampening action when trim valve assembly 52′ is closed, apressure relief valve R1 may be provided in the passage 74 to relieveany overpressure in trim chamber 35 and a similar pressure relief valveR2 may be provided in trim piston 36 to relieve any overpressure in trimchamber 34.

[0081] As it may be desirable to completely deactivate stabilizer 10 inthe event of a failure of a power steering system, a switch 159 forinterrupting electrical power to a solenoid actuated air dump valve 158between an adapter 205 and the air supply line 170 may be provided forvehicles with power steering systems. Switch 159 is mounted on apressure sensor 157 located in a hydraulic line 51 in fluidcommunication with the outlet of the power steering pump (not shown). Aloss of pressure at the pump outlet causes switch 159 to open, therebycausing gas dump valve 158 to open for depressurizing gas chambers 86and 83, which in turn depressurizes liquid reservoirs 87 and 84,resistance chambers 55 and 56 and trim chambers 34 and 35. Dump valve158 is preferably a three-way valve that shuts off the passage inadapter 205 when it opens line 170 to ambient via a vent line 211. Valve158 and vent line 211 are preferably of larger capacity than gas supplyconduit 170 and adapter 205 combined to ensure rapid depressurization ofchambers 83 and 86 upon the opening of pressure switch 159, even if theair passage through the adapter is not completely shut off.

[0082] Referring now to FIGS. 9, 10 and 11, there is shown anothermodification of the invention wherein the air actuated trim valveassembly 52 in the intermediate head of FIG. 2 has been replaced by anair actuated trim valve assembly 52″ in a modified accumulator assembly175 having a valve member 212. Since the components of the air sourceand the control, trim and resistance assemblies are essentially the sameas for the embodiment of FIG. 2, except for a modified intermediate head50″, the same numerical designations have been used for the essentiallysame components of these assemblies.

[0083] The accumulator assembly, generally designated 175, includes aninner low pressure section 176 defined by an inner cylindrical wall 177and an outer annular high pressure section 178 defined by the inner wall177 and a concentric outer cylindrical wall 179, the walls 177 and 179being secured between a cover 180 and a base 181. As may be seen inFIGS. 10 and 11, the inner section is filled with a liquid through afill tube 182 to create a low pressure gas chamber 183 and a trim liquidreservoir 184, and the outer section is filled with a liquid through afill tube 185 to create a high pressure gas chamber 186 and a resistanceliquid reservoir 187. The chamber 186 is connected to the high pressuregas supply 88 via the gas line 89 and a port 190 in cover 180. The lowpressure chamber 183 is connected to the high pressure chamber 186 via ahigh pressure standpipe 191, a low pressure standpipe 192, and apressure regulator 193 located in a base passage 194 between thestandpipes for reducing the gas pressure from that desired in chamber186 to that desired in chamber 183.

[0084] The high pressure reservoir 187 is connected via a base passage196, a base port 197 and a line 198 to a passage 199 in intermediatehead 50″, and head passage 199 in turn is connected to resistancechambers 55 and 56 as described above for passage 99. The low pressurereservoir 184 is connected via a port in the form of a valve seat 200, abase passage 201 and a line 203 to a passage 204 in intermediate head50″, and head passage 204 in turn is connected to trim chamber 34 viathe annular passage 69 defined by the housing 32 and the trim cylinder33 and leading to the chamber port 70. The low pressure chamber 183 isconnected via a port in the form of a valve seat 206, a base passage 207and a line 209 to a passage 210 in intermediate head 50″, and headpassage 210 in turn is connected to trim chamber 35 via the chamber port73.

[0085] Operation of the low pressure trim section 176 of the accumulatorassembly 175 is controlled by a solenoid actuated valving arrangementthat includes the valve seats 200 and 206. These seats are opened andclosed by the valve member 212 that is connected by a rod 213 to a valvepiston 214 that actuates the valve member to its closed position whenhigh pressure air is fed to a pressure chamber 219 above the pistonthrough passages 215 and 216 in the accumulator cover when a solenoid260 is in its activated condition. Solenoid 260 is momentarilydeactivated for a trimming adjustment by the remote electrical switch 62as described below. The valve member 212 has a slot 234 (FIG. 10) forreceiving the low pressure standpipe 192 such that the valve memberreciprocates along this standpipe as it moves between its open andclosed positions.

[0086] Although switch 62 may be hard wired to the solenoid, it ispreferable a remote switch that may be mounted on the steering wheel 38of a motor vehicle for generating a radio signal to activate thesolenoid via a radio signal receiver 117. The ON position of switch 62activates a signal generating unit 118 that in turn activates the signalreceiver 117 causing it to supply an electrical current from a powersource 63 to the solenoid 260. With this option, the switch 65 of arelay 64 does not close until the receiver section 117 receives an ONsignal from the remote unit 118. Similarly, an OFF signal or the absenceof a signal from unit 118 opens switch 65. As may be seen best in FIG.11, the solenoid 260 is secured to the accumulator cover 180 by anadapter 258 having a valve seat 220 between an inlet passage 221connected to cover passage 215 and a transfer passage 222 connected tothe cover passage 216. Internal to the solenoid is a double endedplunger 259 having at its lower end a valve element 225 that is movedupward away from the valve seat 220 by the solenoid 260 when it isactivated so that the differential between high and low pressure airwill force the main valve member 212 to close the main valve seats 200and 206. In other words, when the solenoid 260 of the air valve isactuated by an electric current supplied thereto by the radio receiver117 in response to the electrical switch 62 being in its ON position,the plunger 259 compresses spring 261 and causes valve element 225 toopen valve seat 220. A differential between high and low pressure airwill then act on the trim valve piston 214 and force the trim valvemember 212 to close the trim valve seats 200 and 206.

[0087] In the absence of electrical power, i.e., when the switch 62 isin its OFF position, solenoid plunger 259 is pushed downward by thecompressed return spring 261 so that valve element 225 closes the valveseat 220 and thereby the flow path for pressurized air to reach thevalve piston 214. When valve seat 220 is closed, the chamber 219adjacent the piston 214 is vented to the atmosphere via the head passage216, the adapter passage 222, and solenoid vent passages 224 and 223through solenoid 260. The venting of chamber 219 allows the differentialbetween trim system pressure and ambient pressure, which may be aided bya compression spring (not shown), to open the valve seats 200 and 206 bypushing away therefrom the piston 214, which in turn pulls awaytherefrom the valve member 212 connected to the piston by rod 213. Thetrim valve assembly 52″ is thereby actuated to its open position. Whenvalve seat 220 is reopened to actuate the valve assembly 52″ to itsclosed position, the vent passage 223 is closed by a valve element 226at the upper end of solenoid plunger 259.

[0088] A plurality of the passages 222 communicate with an annularchannel 227 leading to the cover passage 216 and an annular filter 228is preferably provided in the channel 227 to prevent any contaminatesfrom the solenoid or the vent passage 223 from entering the pressurechamber 219 above the valve piston 214. To insure that there is no fluidcommunication from the high pressure section to the low pressuresection, a bellofram diaphragm 229 extends completely over and seals thetop or actuating surface of the valve piston 214. The diaphragm 229 hasa rib 230 around its outer edge and is held in place by clamping thisrib between the cover 180 and a flanged member 231 defining the upperend of the low pressure chamber 183. The diaphragm also includes anannular loop segment 232 that permits reciprocation of the valve pistonwhile maintaining the desired seal between the piston 214 and the cover180. An annular U-cup seal 233 around the piston 214 insures that thereis no fluid communication from the low pressure chamber 183 to theunderside of the diaphragm 229 when the pressure chamber 219 is openedto the vent passage 223. Appropriate O-ring seals, as represented bysmall black or cross-hatched circles in the drawings, are providedbetween the various components of the invention across which there maybe a pressure differential.

[0089] When trim valve 52″ is in its open position with solenoid 260deactivated (valve seat 220 closed and valves seats 200 and 206 open), afluid, preferably a liquid, is supplied from the trim reservoir 184 tothe trim chambers 34 and 35. Therefore, trim piston 36 is released tomove in trim cylinder 33 away from its previously locked position C1(valve seat 220 open and valve seats 200 and 206 closed) to a newposition C2 as illustrated in FIG. 9.

[0090] Referring now to FIGS. 12, 13 and 14, there is shown anothermodification of the invention that may be more economical to manufacturethan the other versions described above, without significantly affectingperformance. In this embodiment, the air actuated trim valve assembly 52in the intermediate head of FIG. 2 has been replaced by an air actuatedtrim valve assembly 301 in a modified accumulator assembly 275 having avalve member 312. Since the components of the air source and thecontrol, trim and resistance assemblies are essentially the same as forthe embodiment of FIG. 9, the same numerical designations have been usedfor the essentially same components of these assemblies as in FIG. 9.

[0091] The accumulator assembly, generally designated 275, includes asingle pressurized enclosure 278 defined by an outer cylindrical wall279 secured between a cover 280 and a base 281 by a series of bolts 283passing through flanges 282 projecting outward from the cover and thebase. The enclosure 278 is filled with a liquid through a fill tube 285,that is similar to the fill tubes 182 and 185 of FIG. 10, to create asingle gas chamber 285 and a single liquid reservoir 287. The chamber286 is connected to the high pressure gas supply 88 via a gas line 289,a solenoid air valve 292 and a gas line 293. As an alternative, the gasline 289 and valve 292 may instead be connected to the gas line 167 andits related pressure varying components shown in FIG. 8.

[0092] The liquid reservoir 287 is connected via a base passage 296, abase port 297 and a line 298 to the passage 199 in intermediate head50″, and head passage 199 in turn is connected to resistance chambers 55and 56 as previously described. The liquid reservoir 287 is alsoconnected via a port in the form of a valve seat 300, a base passage 299and a line 303 to the passage 204 in intermediate head 50″, and headpassage 204 in turn is connected to trim chamber 34 via the annularpassage 69 defined by the housing 32 and the trim cylinder 33 andleading to the chamber port 70. The liquid reservoir 287 is alsoconnected via a port in the form of a valve seat 306, a base passage 307and a line 309 to the passage 210 in intermediate head 50″, and headpassage 210 in turn is connected to trim chamber 35 via the chamber port73.

[0093] Operation of the trim or locking piston 36 is controlled by theair actuated valve assembly 301 that includes the valve seats 300 and306. These seats are opened and closed by a valve member 312 connectedby a rod 313 to a valve piston 314 that actuates the valve member to itsclosed position when high pressure air is fed to a piston chamber 319above the valve piston. The pressurized air is fed from the air source88 through an air supply line 308 and a solenoid air valve 360 that isnormally held open by actuation of its solenoid to pressurize chamber319. The air from line 308 is fed through a port 357 in a top plate 358that, together with a cylinder 359 and the cover 280, defines the pistonchamber 319 and a lower vented chamber 324. Vented chamber 324 is ventedto ambient pressure via a vent passage 325 and is sealed from pistonchamber 319 by a diaphragm 329. The air pressure feed is interrupted(stopped) upon closure of the air valve 360 when the valve solenoid isdeactivated in response to briefly turning off the remote electricalswitch 62 to trim the vehicle steering system into a new centeredposition.

[0094] The solenoid air valve 292 in the main airline 289 is preferablya normally open valve when its solenoid is deactivated and is hard wiredto a separate manually actuated switch 295 for use primarily duringmaintenance when it may be turned on to activate the solenoid and closethe valve, thereby isolating the accumulator enclosure 278 from the airpressure source 88 during accumulator maintenance. When the solenoid ofvalve 292 is deactivated, the valve plunger (not shown) is held open bya compression spring positioned to push the valve member away from thevalve seat, i.e., the spring is positioned opposite to that of spring 61of valve 109 in FIG. 7. Due to this arrangement, full fluid pressure isalways available in the enclosure 278 for operation of the trim assembly28 and the resistance assembly 30 during vehicle operation.

[0095] Except for the difference in valve 292 described above, valves292 and 360 may be essentially the same as valve 109 shown in FIG. 7 andhave respective vent lines 294 and 361 for dumping to ambient the airpressure in respective chambers 286 and 319 when these air valves areclosed to isolate these chambers from the pressurized air source 88. Theline 293 upstream of valve 292 may be connected directly to thepressurized air source 88 as shown in FIG. 12, or via the line 167 andits associated variable pressure system shown in FIG. 8.

[0096] Although switch 62 may be hard wired to the solenoid of valve360, it is preferable a remote switch that may be mounted on thesteering wheel 38 of a motor vehicle for generating a radio signal toactivate the solenoid via a radio signal receiver 117. The ON positionof switch 62 activates the signal generating unit 118 that in turnactuates the signal receiver 117 causing it to supply an electricalcurrent from a power source 63 to the valve solenoid in the same mannerthat solenoid 60 is actuated in the embodiment of FIG. 2.

[0097] In the absence of electrical power, i.e., when the respectiveswitches 295 and 62 are in their OFF positions, the solenoid plunger(not shown) of valve 292 is moved by its compression spring away fromits corresponding main valve seat to open it and the flow path forpressurized air to reach the chamber 286, and the solenoid plunger (notshown) of valve 360 is moved by its compression spring against itscorresponding main valve seat to close it and the flow path forpressurized air to reach the chamber 319.

[0098] When the main seats of each of the valves 292 and 360 are open,the respective vent lines 294 and 361 are closed by vent valve elementsand seats (not shown) adjacent the ends of the solenoid plungersopposite to the ends adjacent the main valve elements and seats, theplungers, valve elements and internal seats and passages beingessentially the same as for valve 109 shown in FIG. 7. On the otherhand, when the valves 292 and 360 are closed, the chambers 286 and 319are vented to the atmosphere via the respective vent lines 294 and 361in the same way that chamber 119 of FIG. 6 is vented through the ventpassage 123 of valve 109 in FIG. 7.

[0099] To insure that there is no fluid communication from the highpressure chamber 319 to the low pressure chamber 324, a belloframdiaphragm 329 extends completely over and seals the top or actuatingsurface of the valve piston 314. The diaphragm 329 has an annular rib330 around its outer edge and is held in place by clamping this rib inan annular groove 327 between the top plate 358 and the cylinder 359defining the piston chamber 319. The diaphragm also includes an annularloop segment 332 that permits reciprocation of the valve piston 314while maintaining the desired seal between the piston and the top plate.An annular U-cup seal 328 around the piston rod 313 insures that thereis no fluid communication from the main pressure chamber 286 to theunderside of the diaphragm 329 that is exposed to ambient pressure vialow pressure chamber 324 and vent passage 325. Appropriate O-ring seals,as represented by small black circles in the drawings, are providedbetween cylinder 359 and cover 280, cover 280 and cylinder 279, andcylinder 279 and base 281, across the interfaces of which there may be apressure differential.

[0100] The venting of chamber 319 via valve 360 allows compressionsprings 322, 322 acting against the underside of valve member 312 toopen the valve seats 300 and 306 by pushing away therefrom the valvemember 312 as shown in FIG. 14. When trim valve 301 is thereby in itsopen position because the solenoid of valve 360 is deactivated, a fluid,preferably a liquid, flows between the reservoir 287 and the trimchambers 34 and 35. Therefore, trim piston 36 is released to move intrim cylinder 33 away from its previously locked position C1 to a newposition C2 as illustrated in FIG. 9 and described above in connectiontherewith.

[0101] The opening and closing movement of valve member 312 is guided bya guide plate 320 and by a guide pin 321 to keep the small cylindricalvalve elements or pads 316 and 318 properly aligned with the valve seats300 and 306, respectively, to completely seal the same when the valvemember 312 is in its closed (down) position. Plate 320 is secured to thecylinder base 281 and has an upstanding flange or guide fin 323 thatreciprocates in a guide channel 326 in the body of the valve member 312.Pin 321 is secured to the cylinder base 281, passes through an aperture317 (FIG. 13) in plate 320 and reciprocates in a guide bore 315 in thebody of the valve member 312. The fin 323 also functions as a flowdivider to keep separate and smooth, i.e., with little or no turbulence,the currents of liquid flowing into and out of the valve seats 300 and306.

[0102] Persons skilled in the art, upon learning of the presentdisclosure, will recognize that various modifications to the assemblies,and to the components and the elements of the assemblies, of theinvention are possible without significantly affecting their functions.For example, a gas such as air may be used as the fluid in both the trimassembly and the resistance assembly of FIGS. 2, 8, 9 and 12. Also,other components may be substituted for those of the trim and resistanceassemblies disclose herein by way of illustrating the present invention.For example, separate accumulators may be substituted for the combinedaccumulator assemblies shown in the drawings, such as the separateaccumulator arrangement illustrated in the related U.S. application Ser.No. 09/699,520 on a Center Holding Assembly For Vehicle SteeringSystems, the entire contents of which is incorporated herein byreference.

[0103] Similarly, other pressurizing assemblies utilizing the same orother fluids may be substituted for the hydraulic accumulators shown.One such alternative is to replace the single pressure regulator betweenthe two accumulator sections 76 and 78 with dual pressure regulators indual gas supply lines that are respectively connected to the low andhigh pressure sections 76 and 78. A second such alternative is to usethe single joint accumulator of FIG. 12 in combination with dualpressure regulators placed directly in the fluid supply lines 98 and 103that are respectively connected to the resistance and trim assemblies.In this second alternative, a pressurized gas instead of a pressurizedliquid could be used as the working fluid in both the trim assembly 28and the resistance assembly 30. Accordingly, while the preferredembodiments have been shown and described in detail by way of example,further modifications and embodiments are possible without departingfrom the scope of the invention as defined by the claims set forthbelow.

What is claimed is:
 1. An apparatus for holding at a center position atleast one steerable member mounted on a frame means for movement toeither side of the center position, said apparatus comprising linkagemeans of variable length extending between said steerable member andsaid frame means, the length of said linkage means defining said centerposition and said linkage means comprising resistance means forproviding a resistance force resisting steering forces tending to movesaid steerable member to either side of the center position, and trimmeans for transmitting said steering forces to said resistance means;wherein said trim means comprises: a trim piston, trim cylinder meansproviding a portion of said linkage means and cooperating with said trimpiston to form first and second trim chambers one on each side of saidtrim piston, and trim fluid means for providing a flow of trim fluid toand from each of said trim chambers and comprising trim valve meansoperable between a closed position for preventing said trim fluid flowso that said trim piston is held in a locked position, and an openposition for allowing said trim fluid flow so that said trim piston isfree to move away from said locked position in said trim cylinder, saidtrim piston movement causing trim fluid flow to one of said trimchambers and trim fluid flow from the other of said trim chambers topermit trimming movement of said steerable member to another centerposition by changing the length of said linkage means in response tosaid steering forces; wherein said resistance means comprises,resistance cylinder means providing a portion of said linkage means,resistance piston means cooperating with said resistance cylinder meansto define first and second resistance chambers, and being arranged formovement in a first direction for compressing a fluid within said firstresistance chamber and for movement in a second direction forcompressing a fluid within said second resistance chamber, resistancerod means providing a portion of said linkage means and arranged formovement with said resistance piston means to either side of a neutralposition corresponding to said center position, and resistance fluidmeans for providing a pressurized resistance fluid in each of said firstand second resistance chambers so that fluid pressure provides saidresistance force by causing said piston means to be biased toward saidneutral position, said bias opposing movement of said steerable membertoward either side of said center position; wherein said trim fluidmeans comprises: trim accumulator means having a trim reservoir forholding a supply of said trim fluid, and means for pressurizing the trimfluid held in said reservoir, and trim conduit means for providing fluidflow between said trim chambers and said trim reservoir when said trimvalve means is in its open position; and, wherein said resistance fluidmeans comprises: resistance accumulator means having a resistancereservoir for holding a supply of said resistance fluid, and means forpressurizing the resistance fluid held in said reservoir, and resistanceconduit means for providing fluid flow between said resistance chambersand said resistance reservoir when said resistance piston means movesaway from said neutral position in said resistance cylinder means.
 2. Anapparatus according to claim 1, wherein said trim conduit meanscomprises a first conduit for communicating with said trim reservoir tosupply trim fluid to said first trim chamber, a second conduit forcommunicating with said trim reservoir to supply trim fluid to saidsecond trim chamber, a first port for providing fluid communicationbetween said trim reservoir and said first conduit, and a second portfor providing fluid communication between said trim reservoir and saidsecond conduit; and wherein said trim valve means is arranged forsimultaneously closing and simultaneously opening both of said first andsecond ports.
 3. An apparatus according to claim 2, wherein said firstport comprises a first valve seat and said second port comprises asecond valve seat; and wherein said trim valve means further comprises:a valve member arranged for reciprocating movement between a closingposition for causing simultaneously closure of said first and secondvalve seats and an opening position for causing simultaneously openingof said first and second valve seats, a piston member connected to saidvalve member for causing said valve member movement in response toclosing and opening pressure differentials across said piston member,and differential pressure means for providing said pressuredifferentials.
 4. An apparatus according to claim 3, wherein saidresistance accumulator means is pressurized to a higher pressure thansaid trim accumulator means, and wherein said trim valve means furthercomprises a solenoid valve means actuatable between a first position forcausing resistance accumulator pressure to provide said closing pressuredifferential and a second position for causing trim accumulator pressureto provide said opening differential pressure.
 5. An apparatus accordingto claim 4, wherein said trim valve means further comprises a thirdconduit means connected to a resistance gas chamber of said resistanceaccumulator means for supplying resistance gas to a resistance gas sideof said piston member, a fourth conduit means connected to a vent spacefor receiving resistance gas vented from the resistance gas side of saidpiston member, a third valve seat leading to said third conduit means, afourth valve seat leading to said fourth conduit means, and a plungermember movable to simultaneously open said third valve seat and closesaid fourth valve seat in response to actuation of said solenoid valvemeans to its first position and movable to simultaneously close saidthird valve seat and open said fourth valve seat in response toactuation of said solenoid valve to its second position.
 6. An apparatusaccording to claim 3, wherein said resistance accumulator pressure actson a resistance gas side of said piston member and said trim accumulatorpressure acts on a trim gas side of said piston member, and wherein aflexible diaphragm is provided on said resistance gas side to preventfluid communication between said sides.
 7. An apparatus according toclaim 1, wherein said trim fluid is a trim liquid, and wherein said trimaccumulator means comprises: a trim reservoir for holding a supply ofsaid trim liquid, a trim gas chamber for pressurizing the trim liquidheld in said trim reservoir; and means for pressurizing said trim gaschamber with a gas to pressurize the trim liquid in said trim reservoir,said pressurizing means permitting the pressure of said gas to be variedso as to vary the liquid pressure.
 8. An apparatus according to claim 7,wherein said resistance fluid is a resistance liquid, and wherein saidresistance accumulator means comprises: a resistance reservoir forholding a supply of said resistance liquid, a resistance gas chamber forpressurizing the resistance liquid held in said resistance reservoir;and means for pressurizing said resistance gas chamber with a gas topressurize the resistance liquid in said resistance reservoir, saidpressurizing means permitting the pressure of said gas to be varied soas to vary the liquid pressure.
 9. An apparatus according to claim 8,wherein said resistance gas chamber is pressurized to a higher pressurethan said trim gas chamber, and wherein said trim pressurizing meanscomprises: said resistance gas chamber, accumulator conduit means fortransferring pressurized gas from said resistance gas chamber to saidtrim gas chamber, and pressure reducing means associated with saidaccumulator conduit means for reducing the pressure of said transferredgas from the resistance chamber pressure to the trim chamber pressure.10. An apparatus according to claim 9, wherein said accumulator conduitmeans comprises a standpipe in said resistance reservoir connected to astandpipe in said trim reservoir for transferring gas through theliquids in said reservoirs.
 11. An apparatus according to claim 9,wherein the chamber and reservoir of said resistance accumulator meanshave annular cross sections and are arranged concentrically around thechamber and reservoir of said trim accumulator means.
 12. An apparatusaccording to claim 1, wherein said resistance piston means comprises afirst resistance piston cooperating with said resistance cylinder meansto define said first resistance chamber, and a second resistance pistoncooperating with said resistance cylinder means to define said secondresistance chamber, each of said first and second resistance pistonsbeing arranged to move independently of the other between a retractedposition and a position for compressing a fluid within its correspondingchamber; and, wherein said resistance rod means comprises a rod headarranged for movement to either side of said neutral positioncorresponding to said center position, and an elongated rod for causingsaid rod head to engage each of said pistons independently of the otherso that movement of said rod head away from said neutral position to oneside causes compressive movement of said first piston without movingsaid second piston and movement of said rod head away from said neutralposition to the other side causes compressive movement of said secondpiston without moving said first piston, resistance fluid pressure ineach of said first and second resistance chambers providing saidresistance force by causing said pistons to bias said rod head towardsaid neutral position, and said bias opposing movement of said steerablemember toward either side of said center position.
 13. An apparatusaccording to claim 1 further comprising operating means for operatingsaid trim valve means between said open and closed positions in responseto a remote input, and control means for providing the remote input tosaid operating means, and wherein said trim valve means is in its openposition until said remote input is provided to said operating means.14. An apparatus according to claim 13, wherein said operating means isactuatable in response to a radio signal, and wherein said control meanscomprises signal means activatable at a location remote from saidoperating means for providing said radio signal thereto.
 15. Anapparatus according to claim 2, wherein said trim valve means furthercomprises: an actuator comprising an actuating member operable between afirst position and a second position in response to a remotely generatedsignal, and a valve member connected to said actuating member andarranged for reciprocating movement between a closing position forcausing simultaneously closure of said first and second ports and anopening position for causing simultaneously opening of said first andsecond ports, said closing position corresponding to the first positionof said actuating member and said opening position corresponding to thesecond position of said actuating member.
 16. An apparatus according toclaim 3, wherein said trim valve means is located in said trimaccumulator means.
 17. An apparatus according to claim 3, wherein saidtrim cylinder means and said resistance cylinder means areinterconnected by an intermediate head member, and wherein said trimvalve means is located in said intermediate head member.
 18. Anapparatus for holding at a center position at least one steerable membermounted on a frame means for movement to either side of the centerposition, said apparatus comprising linkage means of variable lengthextending between said steerable member and said frame means, the lengthof said linkage means defining said center position, and accumulatormeans for holding a supply of fluid and means for pressurizing the fluidheld in said accumulator means; wherein said linkage means comprisesresistance means for providing a resistance force resisting steeringforces tending to move said steerable member to either side of thecenter position, and trim means for transmitting said steering forces tosaid resistance means; wherein said trim means comprises: a trim piston,trim cylinder means providing a portion of said linkage means andcooperating with said trim piston to form first and second trim chambersone on each side of said trim piston, and trim fluid means for providinga flow of trim fluid between said accumulator means and each of saidtrim chambers and comprising trim valve means operable between a closedposition for preventing said trim fluid flow so that said trim piston isheld in a locked position, and an open position for allowing said trimfluid flow so that said trim piston is free to move away from saidlocked position in said trim cylinder, said trim piston movement causingtrim fluid flow to one of said trim chambers and trim fluid flow fromthe other of said trim chambers to permit trimming movement of saidsteerable member to another center position by changing the length ofsaid linkage means in response to said steering forces; wherein saidresistance means comprises: resistance cylinder means providing aportion of said linkage means, resistance piston means cooperating withsaid resistance cylinder means to form first and second resistancechambers, and being arranged for movement in a first direction forcompressing a fluid within said first resistance chamber and formovement in a second direction for compressing a fluid within saidsecond resistance chamber, resistance rod means providing a portion ofsaid linkage means and arranged for movement with said resistance pistonmeans to either side of a neutral position corresponding to said centerposition, and resistance fluid means for providing a pressurizedresistance fluid in each of said first and second resistance chambers sothat fluid pressure provides said resistance force by causing saidresistance piston means to be biased toward said neutral position, saidbias opposing movement of said steerable member toward either side ofsaid center position; and wherein said trim valve means comprises: afirst conduit for communicating with said accumulator means to supplytrim fluid to said first trim chamber, a second conduit forcommunicating with said accumulator means to supply trim fluid to saidsecond trim chamber, a first port for providing fluid communicationbetween said accumulator means and said first conduit, a second port forproviding fluid communication between said accumulator means and saidsecond conduit, and a valve member arranged for reciprocating movementbetween a closing position for causing simultaneously closure of saidfirst and second ports and an opening position for causingsimultaneously opening of said first and second ports.
 19. An apparatusaccording to claim 18, wherein said trim valve means further comprises apiston member connected to said valve member for causing said valvemember movement in response to closing and opening pressuredifferentials across said connected piston member, and differentialpressure means for providing said pressure differentials.
 20. Anapparatus according to claim 19, wherein said differential pressuremeans comprises actuating means for providing said closing pressuredifferential in response to the opening of a solenoid valve in a fluidconduit to pressurize a fluid on one side of said connected piston, andfor providing said opening pressure differential in response to theclosure of said solenoid valve to remove fluid pressure from said onepiston side.
 21. An apparatus according to claim 18, wherein said trimvalve means further comprises actuating means for causing said valvemember closing movement in response to activation of a solenoid and saidvalve member opening movement in response to deactivation of saidsolenoid.
 22. An apparatus according to claim 18, wherein said trimvalve means further comprises actuating means for causing said valvemember movement in response to closing and opening signals provided to asolenoid of said actuating means, and wherein said apparatus furthercomprises signal means for providing said closing and opening signals.23. An apparatus according to claim 18, wherein said trim valve meansfurther comprises an actuating member operable between a first positionand a second position in response to a remotely generated signal, saidclosing position corresponding to the first position of said actuatingmember and said opening position corresponding to the second position ofsaid actuating member.
 24. An apparatus according to claim 18, whereinsaid trim valve means is located in said accumulator means.
 25. Anapparatus according to claim 18, wherein said trim cylinder means andsaid resistance cylinder means are interconnected by an intermediatehead member, and wherein said trim valve means is located in saidintermediate head member.
 26. An apparatus according to claim 18,wherein said at least one steerable member is movable through apredetermined maximum turning angle to either side of said centerposition, and wherein said trim piston has a stroke within said trimcylinder means sufficient to accommodate at least about 30 percent ofsaid turning angle without a compressive movement of said resistancepiston means when said trim valve means is in its open position.